Engines having compressors for intake air, such as turbochargers or superchargers are well known. Compressor efficiency over a large range of operation is desired for economical and efficient operation of the engine. All compressors are limited by a “surge” condition and a “choke” condition. Compressor surging occurs at high pressure ratios and low airflows for a given rotational speed of a compressor wheel, and is shown as a surge line on a compressor map. When surging occurs, the compressor begins operating in a region of diminishing returns and more power input is required to continue operating the compressor while the compressor progressively provides less air at a lower pressure. Compressor choking occurs at higher air flow conditions for a given rotational speed. Compressor choking represents the limit of the compressor air flow capability, and is shown as a choke line on a compressor map. The region between the surge line and the choke line defines the useable range of operation for the compressor.
There have been many attempts to increase the useable range of operation for compressors and thereby allow a single compressor to operate efficiently over a wider range of airflows and speeds. One prevalent solution is the addition of a recirculation chamber at the inlet of the compressor, along with inlet and outlet slots at the inlet of the compressor. This solution has been somewhat successful in extending the service range of the compressor before surging occurs.
One example of a known configuration for a recirculation system in a compressor can be found in the description and accompanying figures of U.S. Pat. No. 4,743,161 by Fisher et al., the contents of which are incorporated herein in their entirety by reference. In Fisher's disclosure, at least one aperture or a series of radial holes are formed in the surface of the inner wall of the compressor housing adjacent to the compressor wheel and are disposed in fluid communication with an annular passage around the inlet bore. Air flowing radially outward from the compressor wheel during operation enters the annular passage and does not interfere with incoming air, thus allowing the compressor to handle higher air flows without surging.
One main disadvantage of the Fisher configuration for a recirculation system is that the improvements to the flow capability of the compressor are limited and are inadequate in meeting the requirements of modern engine systems. Typical modern engine systems resort to use of multiple turbochargers in series or parallel configurations and/or auxiliary compressors to achieve an acceptable performance and efficiency over larger operating ranges.
Accordingly, there is a need for an improvement to existing compressor designs that allows for an extended range of operation of the compressor and the engine.